1. Field of the Invention
The present invention is directed to a liquid crystal cell having a film on a surface of at least one of the carrier plates which film promotes homeotropic or tilted homeotropic orientation of the liquid crystal molecules and to the method of producing the film on the surface of the carrier plates.
2. The Prior Art
Liquid crystal cells having a liquid crystal material such as a nematic material disposed in a layer between a pair of carrier plates at least one of which is transparent have been suggested for a display screen of a display device. If so called DAP effect (deformation of aligned phases) is being utilized in the liquid crystal display screen, an initial homeotropic orientation of the liquid crystal molecules is required. The homeotropic orientation is defined as the liquid crystal molecules being oriented perpendicularly to a plane of the surface of the carrier plate. Such an orientation is necessary in a liquid crystal device which is located between a pair of cross polarizers. In this arrangement, the linearly polarized light entering the liquid crystal film will experience no change in polarization direction and is, thus, consequently completely blocked by the second polarizer. When an electric field is applied to the liquid crystal film, the state of the liquid crystal molecules is changed as a function of the magnitude of the electric field and the polarization of the light entering the liquid crystal is changed or destroyed in relation to the magnitude of change of the orientation of the liquid crystal molecules so that light will pass the second polarizer. The amount of light or intensity of the light passing through the second polarizer is dependent on the magnitude of the electrical field applied to the cell.
To obtain a homeotropic orientation of the liquid crystal molecules, a film of orientation agent such as either lecithin, APAP (anisylidene-p-aminophenol) or PEBAP (p-ethoxy-benzylidene-p'-aminophenol) can be applied to a surface of the carrier plate which is being contacted by the liquid crystal material. A discussion of the application of this type of film is found in an article by Uchida et al, Japanese Journal of Applied Physics, Vol. 11, No. 10, 1972, pages 1559-1565. The orientation effect of these films is explained by the polar interaction between the specific orientation agent and the specific liquid crystal molecules. Thus, the liquid crystal must be matched to the particular orientation agent. The orientation agent or film will produce a change in the layer. The change which depends on the liquid crystal molecule and the particular orientation agent may be either an increased or decreased of the conductivity of the layer.
This type of orientation agent used for the film will decompose at high temperatures. Thus, when the liquid crystal cells are being sealed by the use of fused glass, problems occur with this type of agent.
If a surface of the carrier plate is provided with a structure of matrix-like pattern of grooves by a mechanical process, a homeotropic orientating effect is exerted on the molecules of the liquid crystal material. This orientating effect is discussed by D. W. Berreman, Physical Review Letters, Vol. 28, 1972, pages 1683-1686 and the discussion of this effect is with reference to the elastic continuum theory.